Chapter 11 Part 1 - The Auditory System

Question 1

What is sound?

Answer 1

Audible variations in air pressure.

Question 2

What does the frequency of sound tell us?

Answer 2

The number of compressed or rarefied patches of air that pass by our ears each second.

Question 3

What is the difference between high-frequency and low-frequency sound?

Answer 3

High-frequency sound has more compressed and rarefied regions packed in the same space as low-frequency sound.

Question 4

What is sound intensity? How do we perceive it?

Answer 4

Sound intensity is the air pressure difference between the peaks and the troughs of sound waves. High-intensity waves are perceived as louder.

Question 5

What is the range of the human auditory system?

Answer 5

From 20 Hz to 20.000 Hz. This range decreases significantly, especially at the higher end, as people age and expose themselves to loud sound.

Question 6

What is sound pitch and how does it show?

Answer 6

The pitch of the sound is determined by the frequency. An organ can play sounds as low as 20 Hz, whereas a piccolo can play sounds of 10.000 Hz.

Question 7

What gives music instruments and human voices their unique qualities?

Answer 7

The simultaneous combination of different frequency waves at different intensities.

Question 8

What is the pinna of the ear?

Answer 8

The cartilage formed by skin outside the ear, that creates a sort of a funnel. It helps collect sounds from a wider area.

Question 9

What is the auditory canal?

Answer 9

The entrance to the internal ear. Extends about 2,5 cm into the skull.

Question 10

What is the tympanic membrane?

Answer 10

The end of the auditory canal – also called the eardrum.

Question 11

What are ossicles?

Answer 11

A series of bones connected to the medial surface of the tympanic membrane. They are the smallest bones of the body.

Question 12

Where are ossicles located and what do they do?

Answer 12

They are located in a small air-filled chamber on the inner side of the tympanic membrane. They transfer movements of the tympanic membrane into movements of a second membrane covering a hole in the bone of the skull, called the oval window.

Question 13

What is the oval window?

Answer 13

A hole in the bone of the skull where the movements of the tympanic membrane are transferred.

Question 14

What is the cochlea?

Answer 14

A fluid-filled region that contains the apparatus for transforming the physical motion of the oval window membrane into a neuronal response.

Question 15

What are the 5 steps of the basic auditory pathway?

Answer 15

1. Sound waves move the tympanic membrane.
2. Tympanic membrane moves the ossicles.
3. Ossicles move the membrane at the oval window.
4. Motion at the oval window moves fluid in the cochlea.
5. Movement of fluid in the cochlea causes a response in sensory neurons.

Question 16

How are the areas of the outer ear, the middle ear, and the inner ear divided?

Answer 16

1. The outer ear: From pinna to the tympanic membrane.
2. Middle ear: Tympanic membrane and the ossicles.
3. Inner ear: Apparatus medial to the oval window.

Question 17

What happens after a neural response to sound is generated in the inner ear?

Answer 17

1. The signal is then transferred to and processed by a series of nuclei in the brain stem.
2. The output from these nuclei is sent to a relay in the thalamus, the medial geniculate nucleus (MGN).
3. The MGN projects to primary auditory cortex, or A1, located in the temporal lobe.

Question 18

Name the ossicles and their locations.

Answer 18

1. Malleus / hammer: attached to the tympanic membrane.
2. Incus / anvil: Bone with a rigid connection to malleus, and a flexible connection with stapes.
3. Stapes / stirrup: Third bone of the ossicles, with the flat bottom portion (footplate) moving in and out like a piston at the oval window.

The movements of the footplate transmit sound vibrations to the fluids of the cochlea in the inner ear.

Question 19

What is the Eustachian tube?

Answer 19

A tube, through which the air in the middle ear is continuous with the air in the nasal cavities, although a valve usually keeps this tube closed.

The Eustachian tube can be used to equalize the pressure between the middle ear and outside air, by swallowing or yawning.

Question 20

How and why is the pressure at the tympanic membrane amplified?

Answer 20

By the lever mechanisms of the ossicles, and because the pressure needs to be greater on the oval window than on the tympanic membrane; otherwise most of the sound would disappear. The cochlea is, after all, filled with fluid and not air.

Question 21

What is the attenuation reflex?

Answer 21

Diminishing of sound conduction to the inner ear. It is caused by the tensor tympani and stapedius muscles, which make the chain of ossicles more rigid when they contract. The onset of loud sound causes this reflex.

Question 22

Why does the attenuation reflex not fully protect your hearing?

Answer 22

The reflex has a delay of 50-100msec from the time the sound reaches the ear; thus, it does not protect from very sudden loud sounds. The cochlea may be damaged by them.

Question 23

How does the attenuation reflex react to low and high frequency noise?

Answer 23

It suppresses low frequencies more than high frequencies, which makes high-frequency sounds easier to discern in an environment with low-frequency noise. This enables us to e.g., understand speech better in a noisy environment.

The attenuation reflex is also thought to activate when we speak, so we do not hear our speech as loud as that of others.

Question 24

How is the cochlea divided into different sections?

Answer 24

It is divided into 3 different fluid-filled chambers:

1. Scala vestibuli
2. Scala media
3. Scala tympani

Question 25

What separates the different scalas?

Answer 25

Reissner’s and Basilar membranes, as follows:

1: Between Scala Vestibuli and Scala Media: Reissner’s Membrane.
2: Between Scala Tympani and Scala Media: Basilar Membrane

Question 26

What is the Organ of Corti?

Answer 26

The Organ of Corti contains the auditory receptor neurons. Hanging over this organ is the tectorial membrane.

Question 27

What is at the base of the cochlea?

Answer 27

Two membrane-covered holes: The oval window, and the round window.

Question 28

What is the perilymph?

Answer 28

The fluid in the scala vestibuli and scala tympani. It has a concentration of 7mM of K+ and 140mM of Na+.

Question 29

What is the endolymph?

Answer 29

The fluid in the scala media. Unusual extracellular fluid that has similar concentrations to intracellular fluid: 150mM of K+ and 1mM of Na+.

Because of the ionic concentration differences and permeability of Reissner’s membrane, the endolymph has a membrane potential about 80 mV more positive than on the perilymph: this is called the endocochlear potential.

Question 30

What is the stria vascularis?

Answer 30

The endothelium lining one wall of the scala media and contacting the endolymph. Active transport processes take place here that cause the difference in ion content in the endolymph.

The stria vascularis absorbs sodium from, and secretes potassium into, the endolymph.

Question 31

Describe the process started by stapes when sound is heard, in the instance that the cochlea and its components were rigid.

Answer 31

The stapes pushes the perilymph into the scala vestibuli, which would cause the perilymph to force the round window to bulge out through first the helicotrema and then the scala tympani.

However, the basilar membrane is flexible and bends in response to sound, so it’s not that simple.

Question 32

How does the basilar membrane respond to sound?

Answer 32

The membrane is 5x wider at the apex than the base, and 100x stiffer at the base than the apex.

When the footplate of the stapes is pushed by sound, perilymph is displaced in the scala vestibuli and endolymph is displaced in the scala media: Reissner’s membrane is very flexible. Sound can also pulll the footplate: it is like a piston.

Question 33

How do the endolymph and basilar membrane work together to process sound?

Answer 33

The endolymph makes the basilar membrane bend near the base, which starts a wave that propagates toward the end. (Snapped rope analogy)

The basilar membrane bends differently depending on the frequency of the sound.
High frequency: stiffer base will vibrate a good deal, wave will not propagate far.
Low frequency: The waves will travel all the way up to the floppy apex of the membrane before the energy is dissipated.

Question 34

What is a place code when talking about the basilar membrane?

Answer 34

A place code describes which parts of the basilar membrane are maximally deformed at different sound frequencies.

Question 35

What is tonopy?

Answer 35

Tonopy is the systematic organization of sound frequency within an auditory system, analogous to retinotopy in the visual system.

Tonotopic maps exist on the basilar membrane and within each of the auditory relay nuclei, the MGN, and the auditory cortex.

Question 36

What is the Organ of Corti and what does it consist of?

Answer 36

It is the point where neurons are involved in auditory sensing. The auditory receptor cells that convert mechanical energy into a change in membrane polarization are in the organ of Corti.

The organ of Corti consists of hair cells, the rods of Corti, and various supporting cells.

Question 37

What are the auditory receptors also called? Why?

Answer 37

Auditory receptors are hair cells, since each has 10-300 hairy-looking stereocilia extending from their top. They are not neurons: they do not have axons and in mammals do not generate action potentials. They are specialized epithelial cells.

The bending of these cilia on the hair cells is the critical event in the transduction of sound into a neural signal.

Question 38

What is the reticular lamina?

Answer 38

A thin sheet of tissue near the hair cells. The hair cells are sandwiched between the reticular lamina and the basilar membrane.

The rods of Corti span these span these two membranes and provide structural support.

Question 39

How are the hair cells divided into inner and outer hair cells?

Answer 39

Inner hair cells: Between the modiolus and the rods of Corti, about 4500 form a single row.
Outer hair cells: Cells farther out than the rods of Corti, there are about 12.000-20.000 in humans, arranged in three rows.

Question 40

How far do the stereocilia reach at the top of the hair cells?

Answer 40

They extend above the reticular lamina into the endolymph. Their tips end either in the gelatinous substance of the tectorial membrane (the outer hair cells) or just below the tectorial membrane (inner hair cells).

Question 41

What is a good memory rule for the organization of elements in the organ of Corti?

Answer 41

The basilar membrane is the base.
The tectorial membrane is the roof.
The reticular is in the middle, holding onto the hair cells.

Question 42

How do hair cells form synapses?

Answer 42

They form synapses on neurons whose cell bodies are located in the spiral ganglion within the mediolus. Spiral ganglion cells are bipolar, with neurites extending to the bases and sides of the hair cells, where they receive synaptic input.

Axons from the spiral ganglion enter the auditory nerve, a branch of the auditory-vestibular nerve (cranial nerve VIII), which projects to the cochlear nuclei in the medulla.

Question 43

How can deafness be treated electronically?

Answer 43

Some forms of deafness can be treated by using electronic devices to bypass the middle ear and hair cells, and activate the auditory nerve axons directly.

Question 44

What elements in the inner ear move when there is motion at the oval window by the stapes?

Answer 44

The entire foundation supporting the hair cells moves. The basilar membrane, rods of Corti, reticular lamina, and hair cells are all rigidly connected.

Question 45

How is the bending of stereocilia converted into neural signals?

Answer 45

When the stereocilia bend in one direction, the hair cell depolarizes, and when they bend in another direction, the hair cell hyperpolarizes.

Question 46

How do the hair cells transduce so small amounts of sound energy that they can?

Answer 46

The tip of each stereocilium has a special type of ion channel that is induced to open and close by the bending of stereocilia. When these mechanosensitive transduction channels are open, an inward ionic current flows and generates the hair cell deceptor potential. We do not know the molecular identity of these channels; there are quite few of them in hair cells – an entire hair cell may have only 100.

Question 47

How do the transduction channels function on the hair cells?

Answer 47

1. A stiff filament called a tip link connects each channel to the upper wall of the adjacent cilium.
2. Displacement of the cilia in one direction increases tip link tension, increasing the rate of channel openings and inflowing K+ current.
3. Displacement of the cilia in the opposite direction has the opposite effect.
4. The entry of K+ into the hair cell causes depolarization, which opens voltage-gated calcium channels.
5. The entry of Ca2+ triggers the release of the neurotransmitter glutamate, which activates the spiral ganglion fibers lying postsynaptic to the hair cell.

Question 48

What is the unusual part of the K+ in hair cells?

Answer 48

The opening of K+ channels hyperpolarizes most neurons, but it depolarizes hair cells. The reason for this is the unusually high concentration of K+ in the endolymph, which yields a K+ equilibrium potential of 0 mV, compared to the typical -80 mV in most neurons.

K+ is also driven to the hair cells because of the 80 mV endocochlear potential; it helps create a 125 mV gradient across the stereocilia membranes.

Question 49

How does the brain pay attention to the inner and outer hair cells?

Answer 49

Even though outer hair cells outnumber inner ones 3 to 1, 95% of spiral ganglion neurons communicate with inner hair cells, and 5% with outer ones.

One inner hair cell may be connected to multiple neurons, whereas one neuron may be connected to multiple outer hair cells.

Question 50

What is the potential purpose of outer hair cells?

Answer 50

They seem to act like tiny motors that amplify the movement of the basilar membrane during low-intensity sound stimuli. This is called the cochlear amplifier.

The motor effect of the hair cells also contributes significantly to the traveling wave that propagates down the basilar membrane.

Question 51

How do the outer hair cells contribute to the output of the cochlea?

Answer 51

They amplify the response of the basilar membrane, causing the stereocilia on the inner hair cell to bend more, and the increased transduction process in the inner hair cells produces a greater response in the auditory nerve.

Without the cochlear amplifier, the peak movement of the basilar membrane would be about 100-fold smaller.

Question 52

What happens if the protein prestin is absent or the gene that codes prestin is elimnated in mice?

Answer 52

The animals are nearly deaf: they are 100-fold less sensitive to sound. This is because prestin is essential for the outer hair cells’ motor function.

Question 53

What do the axons innervate when it comes to auditory pathways?

Answer 53

The dorsal cochlear nucleus and ventral cochlear nucleus, ipsilateral to the cochlea where the axons originated. Each axon branches so that it synapses on neurons in both cochlear nuclei.

Question 54

What are 3 important points about auditory pathways relating to:

1. Other projections and brain stem nuclei
2. Feedback
3. Input to cochlear nuclei

Answer 54

1. Several projections and brain stem nuclei contribute to the auditory pathways.
2. There is extensive feedback in auditory pathways.
3. Each cochlear nucleus receives input from just one ear on the ipsilateral side; all other auditory nuclei in the brain receive input from both ears.
   3b. This explains that brain stem damage can only cause deafness in one ear is if a cochlear nucleus on one side is destroyed.

Question 55

What is the common auditory pathway described in the book?

Answer 55

1. Spiral ganglion
   1b. Auditory nerve
2. Ventral cochlear nucleus
3. Superior olive
   3b. Lateral lemniscus
4. Inferior colliculus
5. MGN
6. Auditory cortex

Question 56

How is stimulus intensity represented in the auditory pathway?

Answer 56

1. The firing rate of neurons
2. The number of active neurons

The loudness we perceive correlates with the number of active neurons in the auditory nerve and through the auditory pathway, and their firing rate.

Question 57

How is tonotopy represented in the auditory pathway?

Answer 57

Auditory nerve fibers connected near the apex of the basilar membrane have low characteristic frequencies, and those connected near the base have high characteristic frequencies.

The location of active neurons is thus one indication of the frequency of sound: this is tonotopy.

Question 58

How is phase locking represented in the auditory pathway?

Answer 58

If a sound wave is thought of as a sinusoidal variation in air pressure, the action potentials are fired at the peaks or troughs or any other specific section of the sinus wave; the frequency of the sound is the same as the frequency of the action potentials. Sometimes the action potentials do not fire at every cycle, but still fire at the same location with the wave.

Phase locking happens at low frequencies, not high frequencies. At very low frequencies, phase locking is used, at low to mid (5 KHz) frequencies, both phase locking and tonotopy are useful, and above 5 KHz, only tonotopy is used to represent sound frequency.

Question 59

How do vertical and horizontal sound localization differ from each other?

Answer 59

Vertical sound localization is quite possible with only one ear, whereas horizontal localization is much harder and requires comparison of the sounds reaching the two ears.

Question 60

What is interaural time delay?

Answer 60

The time difference between a sound reaching one and the other ear.

This can be used to detect the horizontal location of the sound with great accuracy. Localization of continous sounds is harder, but the differences in sound phases of continous sounds can also be distinguished.

Question 61

How do the horizontal localizations of continous high and low frequency sounds compare?

Answer 61

Low frequency sounds are easier to localize since the length of the sound wave is greater. High frequency waves fill a shorter length, so interaural time delay is reduced (a 200 Hz sound wave is 172cm in length, a 20.000 Hz sound wave is 1.7 cm in length – the distance between the ears is roughly 20cm).

Interaural delay is not useful for sounds higher in frequency than about 2000 Hz.

Question 62

What is interaural intensity difference?

Answer 62

A difference that exists between two ears because the head effectively casts a sound shadow. If the sound comes directly from the right, the left ear will hear a singnificantly lower intensity sound. This intensity difference can be used for horizontal localization.

Question 63

What is the duplex theory of sound localization?

Answer 63

At sounds in range of 20-2000 Hz, interaural time delay is used for localization, and for sounds between 2000 Hz - 20.000 Hz, interaural intensity difference is used. Together, these two processes constitute the duplex theory of sound localization.

Question 64

What part of the brain processes the sound localization?

Answer 64

The superior olive: the neurons here receive input from cochlear nuclei on both sides of the brain.

Question 65

What is used for the vertical localization of sound?

Answer 65

The pinna of the ear; the outer funnel-like skin structure. The bumps and ridges and curves are essential for assessing the vertical location of sound.

Question 66

Where is the primary auditory cortex (A1)?

Answer 66

On the superior temporal lobe, composed of 6 layers. In general neurons in this area are relatively sharply tuned for sound frequency and possess characteristic frequencies covering the audible spectrum of sound.

Question 67

How do the auditory and visual pathways react to lesions?

Answer 67

In the visual system, a unilateral cortical lesion of striate cortex leads to complete blindness in one visual hemifield.

In the auditory system, unilateral lesions do not have such great effect; in fact, surprisingly much of auditory function is preserved despite them.

Question 68

What does unilateral loss of A1 cause?

Answer 68

Loss of sound localization abilities.